The present invention relates to systems for restraining rotatable components, and more particularly to thermally operated systems for restraining rotatable components of turboalternators.
Foil bearings are a known type of bearing structure that utilize a thin metal journal lining to support a rotatable shaft and create a hydrodynamic film or air bearing with a working fluid (e.g., xenon gas). For example, certain closed Brayton cycle turboalternators can utilize a turboalternator shaft supported by foil bearings. At operational speeds, the rotating shaft is supported by the fluid pressure of the working fluid and generally does not contact the metal structures of the foil bearings. This means that no wear occurs due to direct physical contact with the rotating shaft during operation, although some contact with metal components of the bearings occurs during startup, shutdown and non-operational periods.
However, foil bearings are susceptible to damage, which can reduce or destroy bearing functionality. For instance, with foil bearings used in turboalternators for spacecraft, the turboalternator may not be used during a launch phase of a flight cycle and may only be activated for operation during a later orbital phase of the flight cycle. Because the launch phase will generally subject turboalternator components to stresses, vibration, displacement and other potential sources of damage, it is desired to restrain rotatable components of the turboalternator to prevent damage to the foil bearings during non-operational phases where a hydrodynamic film is not generated and rotatable components can contact the metal structures of the bearings. Active restraint systems, using solenoid actuators or the like, can be used to restrain the rotating components of the turboalternator during the launch phase, but those active systems contain moving parts that present undesirable reliability risks, especially under conditions of extreme ambient temperature variation that occur in aerospace applications.